The invention relates to an electric lamp provided with a lamp vessel which is sealed in a gaslight manner and has a wail of ceramic material, at least one current supply conductor being connected to an electric element arranged in the lamp vessel, which current supply conductor issues from the lamp vessel to the exterior via a lead-through channel in the wail and comprises a first part with a comparatively high and a second part with a comparatively low melting point, which parts form a welded joint with mutually facing ends.
Such an electric lamp, a high-pressure discharge lamp, is known from U.S. Pat. No. 3,363,133. A wall of ceramic material is understood to mean in the present description and claims a wall made from a refractory material such as monocrystalline metal oxide, for example sapphire, polycrystalline metal oxide, for example translucent gastight aluminum oxide (DGA), yttrium-aluminum garnet (YAG) or yttrium oxide (YOX), or polycrystalline non-oxidic material such as aluminum nitride (A1N). In the known lamp, the wall of the lamp vessel is made of DGA. The lamp vessel constitutes a discharge vessel which encloses a discharge space provided with a filling comprising metal halides. The discharge vessel comprises a tubular part and disc-shaped end parts which close off the tubular part on either side. A current supply conductor issues from the discharge vessel to the exterior at either end through a lead-through channel in each of the end parts. The current supply conductors have a first part made of tungsten, with a melting point of 3680 K, to which a second part made of niobium, with a melting point of 2770 K, is welded. End portions of the first parts extending into the discharge space serve as a pair of electrodes and thus constitute the electric element.
A practical method of achieving a welded joint between the parts is by means of resistance welding (butt welding). The parts are each connected to a pole of a current source and then pressed against one another with their ends. The heat generation in the contact surface between the two parts causes the part with the lower melting point to melt, whereby a welded joint with the other part is created.
The lamp vessel of a lamp with a ceramic lamp vessel is formed by means of (pre)sintering prior to the provision and gaslight fastening of the current supply conductors. In general, the current supply conductor is passed through the lead-through channel with narrow fit, after which a melting-ceramic connection is provided between the lead-through channel and the current supply conductor so as to realise the required gaslight seal of the discharge vessel.
When the parts of the current supply conductor are welded together, a thickening often arises around the boundary between the parts, which prevents the current supply conductor fitting in the lead-through channel--or being passed through the channel. This leads to a comparatively high wastage of current supply conductors, which adversely affects the cost price. Mechanical removal of the thickened portion, such as by grinding, involves an elaborate additional operation.
The thickening may be counteracted in that the welding current is limited or cut off earlier. The wastage percentage is high in that case as well, however, because no reliable weld is achieved in many cases then.